A major component of NASA's next-generation Wide-Field Infrared Survey Telescope (WFIRST) reached a major milestone as its advanced coronagraph or "starglasses" passed its preliminary design review. An essential part of the telescope's exoplanet census mission, its purpose is to block a star's light so that any orbiting planets become visible.
One of the biggest problems with exoplanets is that most of the ones we've found are in orbit around stars. This makes them very hard to study because they are so much dimmer than their parent stars, causing them to get lost in the glare and making direct telescopic observations difficult, if not impossible.
One way to avoid this is by means of a coronagraph. The first coronagraph was invented by French astronomer Bernard Lyot in 1931 as a way to study the Sun's outer atmosphere or corona. Normally, the corona is only visible during a total eclipse when the Moon passes in front of the Sun. Because the corona is an important source of information about the Sun's structure, composition, and dynamics, Lyot came up with a way to create his own artificial eclipses on demand using an opaque disc to block the Sun's light, so the corona becomes visible.
This is essentially what the starglasses for WFIRST do. According to WFIRST project scientist Jason Rhodes, the objective is to reduce the incoming starlight by a factor of a billion while allowing the faint light from the exoplanet to come through. This will let scientists capture both images and spectrographs directly from the larger exoplanets to learn more about their composition and atmosphere.
"With WFIRST we'll be able to get images and spectra of these large planets, with the goal of proving technologies that will be used in a future mission – to eventually look at small rocky planets that could have liquid water on their surfaces, or even signs of life, like our own," says Rhodes.
The WFIRST coronagraph won't be the first one to fly into space. The Hubble Space Telescope has a much less sophisticated one and the still-to-fly James Webb Space Telescope is more advanced, but won't have the starlight suppression capabilities of WFIRST.
"WFIRST should be two or three orders of magnitude more powerful than any other coronagraph ever flown,", Rhodes said. "There should be a chance for some really compelling science, even though it's just a tech demo."
The new coronagraph works by means of a series of complex discs and optical stops working with two flexible mirrors that are mounted on thousands of piston-like actuators. These change the shape of the reflectors in real time to compensate for any flaws in the telescope's optics on the scale of a width of a strand of DNA. Meanwhile, the optical masking reduces the incoming starlight by means of diffraction – essentially by having the starlight block itself as the waves interfere with one another.
This greatly reduces the starlight while helping to capture high-resolution exoplanet images. Potentially, this "active wavefront control" could lead to coronagraphs that are 100 to 1,000 times more capable than previous technologies.
Now that the WFIRST coronagraph has passed its design review, it goes on to building the actual mission hardware. The space telescope is due to begin operations in the mid-2020's.
The video below discusses the WFIRST coronagraph.
Source: NASA
